The present invention relates generally to drop-on-demand liquid emission devices, and, more particularly, to ink jet devices which employ thermo-mechanical actuators.
Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in inkjet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or xe2x80x9cbubble jetxe2x80x9d), uses electroresistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421.
Electroresistive heater actuators have manufacturing cost advantages over piezoelectric actuators because they can be fabricated using well developed microelectronic processes. On the other hand, the thermal ink jet drop ejection mechanism requires the ink to have a vaporizable component, and locally raises ink temperatures well above the boiling point of this component. This temperature exposure places severe limits on the formulation of inks and other liquids that may be reliably emitted by thermal ink jet devices. Piezoelectrically actuated devices do not impose such severe limitations on the liquids that can be jetted because the liquid is mechanically pressurized.
The availability, cost, and technical performance improvements that have been realized by ink jet device suppliers have also engendered interest in the devices for other applications requiring micro-metering of liquids. These new applications include dispensing specialized chemicals for micro analytic chemistry as disclosed by Pease et al., in U.S. Pat. No. 5,599,695; dispensing coating materials for electronic device manufacturing as disclosed by Naka et al., in U.S. Pat. No. 5,902,648; and for dispensing microdrops for medical inhalation therapy as disclosed by Psaros et al., in U.S. Pat. No. 5,771,882. Devices and methods capable of emitting, on demand, micron-sized drops of a broad range of liquids are needed for highest quality image printing, but also for emerging applications where liquid dispensing requires monodispersion of ultra small drops, accurate placement and timing, and minute increments.
A low cost approach to micro drop emission is needed which can be used with a broad range of liquid formulations. Apparatus and methods are needed which combines the advantages of microelectronic fabrication used for thermal ink jet with the liquid composition latitude available to piezoelectromechanical devices.
A DOD ink jet device which uses a thermo-mechanical actuator was disclosed by T. Kitahara in JP 20-30543, filed Jul. 21, 1988. The actuator is configured as a bi-layer cantilever moveable within an ink jet chamber. The beam is heated by a resistor causing it to bend due to a mismatch in thermal expansion of the layers. The free end of the beam moves to pressurize the ink at the nozzle causing drop emission. Recently, disclosures of a similar thermo-mechanical DOD ink jet configuration have been made by K. Silverbrook in U.S. Pat. Nos. 6,067,797; 6,234,609; and 6,239,821. Methods of manufacturing thermo-mechanical ink jet devices using microelectronic processes have been disclosed by K. Silverbrook in U.S. Pat. Nos. 6,254,793 and 6,274,056.
DOD ink jet devices using buckling mode thermo-mechanical actuators are disclosed by Matoba et al., in U.S. Pat. No. 5,684,519, and by Abe et al., in U.S. Pat. No. 5,825,383. In these disclosed devices a thermo-mechanical plate, forming a portion of a wall of the ink chamber, is caused to buckle inward when heated, ejecting drops.
Thermo-mechanical actuator drop emitters are promising as low cost devices which can be mass produced using microelectronic materials and equipment and which allow operation with liquids that would be unreliable in a thermal ink jet device. However, operation of thermal actuator style drop emitters, at high drop repetition frequencies, requires careful attention to excess heat build-up. The drop generation event relies on creating a pressure impulse in the liquid at the nozzle. A significant variation in baseline temperature of the emitter device, and, especially, of the thermo-mechanical actuator itself, causes erratic drop emission including drops of widely varying volume and velocity.
Temperature control techniques are known in thermal ink jet systems which use non-drop emitting electrical pulses to maintain a temperature set-point for some element of the thermal ink jet device. Bohorquez et al., in U.S. Pat. No. 5,736,995, discloses a method for operating a thermal ink jet device having a temperature sensor on the same substrate as the bubble-forming heater resistors. Non-printing electrical pulses are applied as needed to the heater resistors, during clock periods when drops are not being commanded, to maintain the substrate temperature at a set-point.
K. Yeung in U.S. Pat. No. 5,168,284 discloses an open loop method for maintaining a constant printhead temperature in a thermal ink jet printhead. Non-printing pulses, having reduced energy with respect to printing pulses, are applied to the heater resistors during all clock periods when print drops are not commanded.
The known temperature control approaches which have been developed and disclosed for thermal ink jet devices are not sufficient for operating a thermo-mechanical actuator drop emitter at high frequencies. The known approaches do not account for the highly complex thermal effects caused by the various heat flows within and away from the thermo-mechanical actuator when pulsed in response to a typical DOD data stream. Drop repetition rates must be severely limited if the thermal history of the thermo-mechanical actuator is not stabilized.
Thermo-mechanical DOD emitters are needed which manage the thermal condition and profiles of device elements so as to maximize the productivity of such devices. The inventors of the present invention have discovered that uniform DOD emission can be achieved at greatly improved frequencies by operating the thermal actuator with particular attention to the steady state flow of heat energy into the actuator, drop emitter device, and overall drop emission apparatus. This approach is unlike prior art thermal ink jet systems which are managed via device substrate temperature control. It is difficult to predict the residual position of a thermal actuator, especially in the case of a large array of thermal actuators, from a measurement of temperature at some other location in the drop emitter device.
It is therefore an object of the present invention to provide a liquid drop emitter which is actuated by a thermo-mechanical means.
It is also an object of the present invention to provide a thermo-mechanical drop emitter to produce series and groups of drops having substantially equal volume and velocity.
It is further an object of the present invention to provide a thermo-mechanical drop emitter by maintaining a constant input energy thereby creating a stable thermal condition in the thermo-mechanical actuator, drop emitter device and apparatus, and enabling operation of the emitter in a drop-on-demand fashion at high frequency.
The foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing a liquid drop emitter for emitting a series of liquid drops having substantially uniform volume and velocity, wherein the drop emitter comprises a liquid-filled chamber having a nozzle and a thermal actuator for applying pressure to liquid at the nozzle. The thermal actuator further comprises electroresistive heater means that suddenly heat the thermal actuator in response to electrical pulses. The sudden heating causes bending of the thermal actuator and pressurization of the liquid at the nozzle sufficient to cause drop ejection. A source of electrical pulses is connected to the liquid drop emitter and a controller means receives commands to emit drops and determines the timing and parameters of the electrical pulses which are applied to the liquid drop emitter. The method of operating comprises the determining a nominal electrical pulse having a nominal energy, E0, and a nominal pulse duration, TP0, wherein said nominal electrical pulse, when applied to the electroresistive means with a repetition period of TC, causes the emission of a drop having a predetermined volume and velocity. The method also comprises determining a steady state electrical pulse having energy E0, and a steady state pulse duration TPSS, wherein said steady state electrical pulse, when applied to the electroresistive means, does not cause the emission or weeping of the liquid from the nozzle. The method further comprises applying to the electroresistive means during every period of time TC, a nominal electrical pulse to emit a drop, or a steady state electrical pulse, so that an average power PAVE, where PAVE=E0/TC, is applied to the liquid drop emitter in order to maintain a steady state thermal condition. The application of steady state electrical pulses may also be suspended to save energy or initiated at system start up based on a determination of the time required to reach a steady state thermal condition and a known master sequence of drop emission commands.
The present invention is particularly useful for liquid drop emitters for DOD ink jet printing. In this embodiment, image data is presented in highly varying clusters and series of drop print commands. The present invention allows a thermo-mechanical actuated ink jet device to accommodate these patterns at high net drop emission frequency.